Solventless two component primer composition for improved adhesion of RTV silicone elastomers to substrates

ABSTRACT

A organic solventless two component rapidly curing primer composition comprising a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl and an organotitanate, a process for the manufacture thereof, and a process for the use therewith said primer particularly suited for use with room temperature vulcanizable silicone sealants and a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl as a primer for room temperature vulcanizable sealants whereby such sealants may be painted.

This is a continuation-in-part of application Ser. No. 08/400,938 filed on Mar. 8, 1995.

FIELD OF THE INVENTION

The present invention relates to a solventless primer composition for improving the adherence of room temperature vulcanizable silicone elastomeric compositions to various substrates. More particularly the present invention relates to a primer composition comprising a low molecular weight linear dimethyl silicone fluid endcapped with an amino-ethyl-amino-propyl dimethoxysilane and an alkyl titanate.

BACKGROUND OF THE INVENTION

In order to adhere room temperature vulcanizable silicone elastomers to various substrates it has usually been necessary to first coat or treat the substrate with a primer. The typical primers employed for this purpose have heretofore contained an organic solvent and a coupling agent. U.S. Pat. Nos. 4,749,741; 4,681,636, and 3,671,483 disclose such primer compositions.

Because of increasing concerns about the release of volatile organic solvents to the atmosphere in various organic products such as paints, sealants, aerosols, and the like and the deleterious effects on the environment of the release of volatile organic compounds into the environment or atmosphere, it has become desirable to minimize the volatile organic content of many commercially useful products. The absence of solvents, which dilute the effective concentration of high molecular weight materials that are also usually viscous renders the simple removal of solvent from formulations a challenging task. This is because the compositions themselves must be re-formulated and frequently there must be a change in the molecular architecture of the compounds employed in order to secure the same benefits of a given intended product while reducing the amount of volatile organic matter present in the formulation.

In the case of sealants and room temperature vulcanizable silicone compositions some progress towards this goal has been made by recent developments in the art. U.S. Pat. No. 5,238,708 discloses a primer suitable for use with room temperature vulcanizable compositions comprising an organotitanate and a tin carboxylate. U.S. Pat. No. 4,332,844 discloses a composition comprising an alkoxy containing silicone compound, an organohydrogensilicon compound, and an organotitanate that improves the adhesion between a silicone rubber and a substrate.

SUMMARY OF THE INVENTION

The present invention relates to a solventless rapidly curing two component primer composition for room temperature vulcanizable silicone elastomers comprising of from about 100.1 parts by weight to about 120 parts by weight of:

(A) 100 parts by weight of a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula:

    (NH.sub.2 (CH.sub.2).sub.2 NH(CH.sub.2).sub.3)Si(OCH.sub.3).sub.2 O--).sub.2 --(Si(CH.sub.3).sub.2 O--).sub.x

where x varies from about 6 to about 12, and;

(B) from about 0.1 to about 20 parts by weight of an organotitanate having the formula:

    Ti(OR).sub.4

where R is a hydrocarbon radical having from 1 to about 8 carbon atoms.

The organotitanate is more preferably selected from the group consisting of tetrabutyl titanate, tetramethyl titanate, tetraisopropyl titanate, ethylmethyldibutyl titanate, tetraoctyl titanate, and tetra-ethylhexyl titanate, and is most preferably tetrabutyl titanate. The range of organotitanate ranges from about 0.1 to about 20 parts by weight in a composition that has a total parts by weight composition of 100.1 parts by weight to about 120 parts by weight, more preferably this range varies from about 0.1 to about 10 parts by weight and most preferably this range varies from about 0.1 to about 5 parts by weight.

The present invention also relates to a process for making a two component rapidly curing primer for room temperature vulcanizable silicone rubber elastomers comprising:

a) reacting a low viscosity dimethylsiloxane oligomer with amino-ethyl-amino-propyl-trimethoxysilane, forming thereby a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula:

    (NH.sub.2 (CH.sub.2).sub.2 NH(CH.sub.2).sub.3)Si(OCH.sub.3).sub.2 O--).sub.2 --(Si(CH.sub.3).sub.2 O--).sub.x

where x varies from about 6 to about 12; and

b) mixing therewith an organotitanate having the formula:

    Ti(OR).sub.4

where R is a hydrocarbon radical having from 1 to about 8 carbon atoms.

The present invention further relates to a process for applying a sealant to a substrate comprising:

a) priming said substrate with a primer comprising of from about 100.1 parts by weight to about 120 parts by weight of:

(A) 100 parts by weight of a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula:

    (NH.sub.2 (CH.sub.2).sub.2 NH(CH.sub.2).sub.3)Si(OCH.sub.3).sub.2 O--).sub.2 --(Si(CH.sub.3).sub.2 O--).sub.x

where x varies from about 6 to about 12, and;

(B) from about 0.1 to about 20 parts by weight of an organotitanate having the formula:

    Ti(OR).sub.4

where R is a hydrocarbon radical having from 1 to about 8 carbon atoms;

b) curing said primer on said substrate; and

c) applying thereto a room temperature vulcanizable silicone sealant.

The present invention further relates to a process for applying a sealant to a substrate comprising:

a) priming said substrate with a primer comprising of from about 100.1 parts by weight to about 120 parts by weight of:

(A) 100 parts by weight of a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula:

    (NH.sub.2 (CH.sub.2).sub.2 NH(CH.sub.2).sub.3)Si(OCH.sub.3).sub.2 O--).sub.2 --(Si(CH.sub.3).sub.2 O--).sub.x

where x varies from about6 to about 12, and;

(B) from about 0.1 to about 20 parts by weight of an organotitanate having the formula:

    Ti(OR).sub.4

where R is a hydrocarbon radical having from 1 to about 8 carbon atoms;

b) curing said primer on said substrate; and

c) applying thereto a room temperature vulcanizable silicone sealant.

The present invention further relates to a curable primer composition that bonds to RTV sealants and presents a paintable surface, said primer comprising:

a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula:

    (NH.sub.2 (CH.sub.2).sub.2 NH(CH.sub.2).sub.3)Si(OCH.sub.3).sub.2 O--).sub.2 --(Si(CH.sub.3).sub.2 O--).sub.x

where x varies from about 6 to about 12.

Thus the present invention relates to a composition of matter, a process for preparing that composition, and a use for that composition.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a solventless two component primer for improving the adhesion of room temperature vulcanizable silicone compositions to substrates comprising an organopolysiloxane endcapped with amino-ethyl-amino-propyl dimethoxysiloxyl and an organotitanate. Further such a composition provides a solventless primer, that contains no volatile organic compounds as formulated, thus providing an environmental benefit for its use as well as providing an improved adhesion to various substrates. It is an additional benefit of the composition of the present invention that it undergoes a rapid hydrolytic cure after being applied to a substrate, such cure being sufficiently rapid so that application of silicone room temperature vulcanizable sealants to a substrate primed with composition of the present invention may proceed immediately after the application of the primer. This is particularly convenient in terms of eliminating a waiting period while the primer cures. By solventless applicants define a concentration of volatile organic matter that is below about 5 weight percent of the total weight of the composition, preferably below about 1 weight percent, and most preferably below about 0.25 weight percent.

Thus we now disclose, a solventless rapidly curing two component primer for room temperature vulcanizable silicone elastomers comprising:

(A) 100 parts by weight of a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula:

    (NH.sub.2 (CH.sub.2).sub.2 NH(CH.sub.2).sub.3)Si(OCH.sub.3).sub.2 O--).sub.2 --(Si(CH.sub.3).sub.2 O--).sub.x

where x varies from about 6 to about 12, and

(B) 0.1 to about 20 parts by weight of an organotitanate having the formula:

    Ti(OR).sub.4

where R is a hydrocarbon radical having from 1 to about 8 carbon atoms. The primer composition thus varies from about 100.1 parts by weight where 100 parts by weight are component (A) and 0.1 parts by weight are parts by weight of component (B) to 120 parts by weight where 100 parts by weight are component (A) and 20 parts by weight are component (B). Compounds which are exemplary of the organotitanate and which are preferred are: tetrabutyl titanate, tetramethyl titanate, tetraisopropyl titanate, ethylmethyldibutyl titanate, tetraoctyl titanate, and tetraethylhexyl titanate. The most preferred titanate is tetrabutyl titanate. The organotitanate is more preferably selected from the group consisting of tetrabutyl titanate, tetramethyl titanate, tetraisopropyl titanate, ethylmethyldibutyl titanate, tetraoctyl titanate, and tetra-ethylhexyl titanate, and is most preferably tetrabutyl titanate. The range of organotitanate ranges from about 0.1 to about 20 parts by weight in a composition that has a total parts by weight composition of 100.1 parts by weight to about 120 parts by weight, more preferably this range varies from about 0.1 to about 10 parts by weight and most preferably this range varies from about 0.1 to about 5 parts by weight.

The present invention also relates to a process for making a two component rapidly curing primer for room temperature vulcanizable silicone rubber elastomers comprising:

a) reacting a low viscosity dimethylsiloxane oligomer with amino-ethyl-amino-propyl-trimethoxysilane, forming thereby a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula:

    (NH.sub.2 (CH.sub.2).sub.2 NH(CH.sub.2).sub.3)Si(OCH.sub.3).sub.2 O--).sub.2 --(Si(CH.sub.3).sub.2 O--).sub.x

where x varies from about 6 to about 12; and

b) mixing therewith an organotitanate having the formula:

    Ti(OR).sub.4

where R is a hydrocarbon radical having from 1 to about 8 carbon atoms.

The present invention further relates to a process for applying a sealant to a substrate comprising:

a) priming said substrate with a primer comprising of from about 100.1 parts by weight to about 120 parts by weight of:

(A) 100 parts by weight of a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula:

    (NH.sub.2 (CH.sub.2).sub.2 NH(CH.sub.2).sub.3)Si(OCH.sub.3).sub.2 O--).sub.2 --(Si(CH.sub.3).sub.2 O--).sub.x

where x varies from about 6 to about 12, and;

(B) from about 0.1 to about 20 parts by weight of an organotitanate having the formula:

    Ti(OR).sub.4

where R is a hydrocarbon radical having from 1 to about 8 carbon atoms;

b) curing said primer on said substrate; and

c) applying thereto a room temperature vulcanizable silicone sealant.

The present invention further relates to a process for applying a sealant to a substrate comprising:

a) priming said substrate with a primer comprising of from about 100.1 parts by weight to about 120 parts by weight of:

(A) 100 parts by weight of a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula:

    (NH.sub.2 (CH.sub.2).sub.2 NH(CH.sub.2).sub.3)Si(OCH.sub.3).sub.2 O--).sub.2 --(Si(CH.sub.3).sub.2 O--).sub.x

where x varies from about. 6 to about 12, and;

(B) from about 0.1 to about 20 parts by weight of an organotitanate having the formula:

    Ti(OR).sub.4

where R is a hydrocarbon radical having from 1 to about 8 carbon atoms;

b) curing said primer on said substrate; and

c) applying thereto a room temperature vulcanizable silicone sealant.

Because this formulation is highly sensitive to the presence of water, undergoing a rapid hydrolyrically catalyzed cure, it should be prepared and stored under essentially anhydrous conditions.

All of the United States Patents referenced herein are hereby and herewith specifically incorporated by reference.

EXPERIMENTAL

Preparation of Dimethylsiloxane Oligomer

A suitable reaction vessel, equipped with an agitator, heater, nitrogen purge, and a vacuum source was charged with 109 parts by weight dry acetone, 10 parts by weight water, and 2 parts by weight Bentonite clay quartz (Super Filtrol grade #20). These ingredients were mixed at a moderate speed for 15 minutes. To this mixture was added 100 parts by weight molten methyl trimer (hexamethyltrisiloxane) and mixed with heating at a temperature ranging from 50° to 60° C. for six hours, at which time the percentage of unreacted trimer had been reduced to less than 5%. At this point, 1.3 parts by weight magnesium oxide and 1.0 parts by weight calcined diatomaceous earth (Celite 545®) were charged to reaction mixture and mixed for 30 minutes. The reaction mixture was cooled to 25° C. and filtered. Acetone was removed from the reaction mixture by stripping under a partial vacuum under conditions where the temperature never exceeded 55° C. This resulted in the isolation of the dimethylsiloxane oligomer. After being cooled to 25° C. the dimethylsiloxane oligomer was analyzed and found to have a silanol level of 5-8 weight percent, a viscosity of 3-10 cps, and a residual trimer level below 5 weight percent. The preparation was colorless.

Preparation of amino-ethyl-amino-propyl-dimethoxy stopped dimethylsiloxane oligomer

A suitable reaction vessel, equipped with an agitator, heater, nitrogen purge, and a vacuum source was charged with 100 parts by weight of dimethylsiloxane oligomer, prepared as previously described, and was heated to 70°-90° C. while stirring. A solution consisting of 42 parts by weight amino-ethyl-amino-propyltrimethoxysilane, 5.1 parts by weight di-butylamine, and 2.4 parts by weight acetic acid was charged to the reaction mixture and mixed for 30 minutes at a temperature of 70°-90° C. under a nitrogen purge. The mixture was cooled to 40°-60° C. and 15 parts by weight hexamethyldisilazane was charged and mixed for 30 minutes. The mixture was placed under vacuum for 30 minutes to de-gas and then cooled to room temperature.

The following experiments were conducted using 1×4 inch panels of anodized aluminum that had been cleaned by washing with isopropanol. A thin film of candidate primer was then applied with a clean dry cloth. Immediately following application of the primer, ASTM-C-794 (peel adhesion specimens) specimens were prepared. The following sealants were tested for cohesive failure after being allowed to cure for ten days on the primed aluminum panels:

Sealant A: A silicone room temperature vulcanizable composition using a tin catalyzed alkoxy cure.

Sealant B: A silicone room temperature vulcanizable composition using a tin catalyzed acetoxy cure.

Sealant C: A silicone room temperature vulcanizable composition using a titanate catalyzed alkoxy cure.

Experiment: 1

Primer: amino-ethyl-amino-propyl-dimethoxy stopped dimethylsiloxane oligomer.

    ______________________________________                                                    Average Pull                                                        Sealant    lb/in.        % Cohesive Failure                                    ______________________________________                                         A          47            30                                                    B          4             0                                                     C          36            90                                                    ______________________________________                                    

Experiment: 2

Primer: 99 parts by weight amino-ethyl-amino-propyl-dimethoxy stopped dimethylsiloxane oligomer and I part by weight tetrabutyl titanate.

    ______________________________________                                                    Average Pull                                                        Sealant    lb/in.        % Cohesive Failure                                    ______________________________________                                         A          28            45                                                    B          4             0                                                     C          46            90                                                    ______________________________________                                    

Experiment: 3

Primer: 97 parts by weight amino-ethyl-amino-propyl-dimethoxy stopped dimethylsiloxane oligomer and 3 parts by weight tetrabutyl titanate.

    ______________________________________                                                    Average Pull                                                        Sealant    lb/in.        % Cohesive Failure                                    ______________________________________                                         A          34            90                                                    B          9             15                                                    C          33            90                                                    ______________________________________                                    

Experiment: 4

Primer: 95 parts by weight amino-ethyl-amino-propyl-dimethoxy stopped dimethylsiloxane oligomer and 5 parts by weight tetrabutyl titanate.

    ______________________________________                                                    Average Pull                                                        Sealant    lb/in.        % Cohesive Failure                                    ______________________________________                                         A          35            95                                                    B          27            70                                                    C          45            100                                                   ______________________________________                                    

Experiment: 5

Primer: none, using unprimed anodized aluminum.

    ______________________________________                                                    Average Pull                                                        Sealant    lb/in.        % Cohesive Failure                                    ______________________________________                                         A          44            40                                                    B          7.5           0                                                     C          21            10                                                    ______________________________________                                    

Experiment 6

At a level of 10 parts by weight tetrabutyl titanate with 90 parts by weight amino-ethyl-amino-propyl-dimethoxy stopped dimethylsiloxane oligomer, the hydrolysis rate increased to the point where substrate priming was not practical by the method used in the previous tests. Other methods of priming presently available or yet to be developed may render higher levels of organotitanate suitable for use in priming formulations.

Explanation of results

Cohesive failure is defined as a splitting of the sealant within the sealant layer itself, substantially down the middle when the sample is pulled to destruction. Adhesive failure is a failure of the sealant to adhere to a substrate when pulled to destruction. Based on the results presented in experiments 1 through 5, while there was essentially no adhesive failure, there was an increased cohesive failure, indicating the success of the primer composition in maintaining adherence of the sealant to the primer and the primer to the substrate. Cohesive failure thus indicates a failure of the sealant within the sealant layer as distinguished from what the sealant may be bonded to in terms of substrate or primer.

Experiment 7

Sheets of RTV sealants were allowed to cure at room temperature for 24 hours. Sheets were prepared from the following RTV sealants:

1) a silicone RTV with a tin catalyzed alkoxy cure (RTV-Sn-Alk),

2) a silicone RTV with a tin catalyzed acetoxy cure (RTV-Sn-Acet), and

3) a silicone RTV with a titanate catalyzed alkoxy cure (RTV-Ti-Alk). A thin film of the priming compound of the present invention, amino-ethyl-amino-propyl-dimethoxy stopped dimethylsiloxane oligomer, was applied to each sheet. Immediately following application of the priming compound, a coat of paint was applied using a paint brush. After the first coat of paint was dried a second coat of paint was applied and allowed to dry. Three types of commercially available paints were used: 1) an exterior flat latex, 2) and exterior semi-gloss latex, and 3) an exterior gloss-oil. The percentage of coverage of the primed RTV sheets was then measured:

    ______________________________________                                                             Coverage Test                                              Sheet    Flat Latex Semi-Gloss Latex                                                                             Gloss-Oil                                    (primed) % coverage % coverage    % coverage                                   ______________________________________                                         RTV-Sn-Alk                                                                              100        100           100                                          RTV-Sn-Acet                                                                             100        100           100                                          RTV-Ti-Alk                                                                              100        100           100                                          ______________________________________                                    

After allowing the paint to cure at room temperature for seven days, a bonding test was performed. The bonding test consisted of scoring the painted surfaces with a razor in parallel 1/4" intervals both horizontally and vertically. A strip of cellophane adhesive tape was then applied to the surface of the scored paint. The tape was peeled off and the percentage of paint remaining on the surface was measured.

    ______________________________________                                                             Bonding Test                                               Sheet    Flat Latex Semi-Gloss Latex                                                                             Gloss-Oil                                    (primed) % remaining                                                                               % remaining   % remaiming                                  ______________________________________                                         RTV-Sn-Alk                                                                              100        100           100                                          RTV-Sn-Acet                                                                             80         84            100                                          RTV-Ti-Alk                                                                              100        100           100                                          ______________________________________                                    

Experiment 8

Experiment 8 is a repeat of experiment except that the silicone RTV sheets were not primed.

    ______________________________________                                                             Coverage Test                                                       Flat Latex Semi-Gloss Latex                                                                             Gloss-Oil                                    Sheet    % coverage % coverage    % coverage                                   ______________________________________                                         RTV-Sn-Alk                                                                              <50        <50           100                                          RTV-Sn-Acet                                                                             <50        <50           100                                          RIV-Ti-Alk                                                                              <50        <50           100                                          ______________________________________                                                             Bonding Test                                                        Flat Latex Semi-Gloss Latex                                                                             Gloss-Oil                                    Sheet    % remaining                                                                               % remaining   % remaining                                  ______________________________________                                         RTV-Sn-Alk                                                                              0          0             13                                           RTV-Sn-Acet                                                                             20         0             0                                            RTV-Ti-Alk                                                                              0          0             19                                           ______________________________________                                    

Experiment 9

This example describes the relationship between paint coverage and the length of time between the application of the priming compound and the application of paint. This test was conducted using sheets of tin catalyzed alkoxy cured RTV silicone.

    ______________________________________                                         Time               Coverage test                                               Interval                                                                              Flat Latex  Semi-Gloss Latex                                                                             Gloss-oil                                     hrs    % remaining % remaining   % remaining                                   ______________________________________                                         0      100         100           100                                           1      100         100           100                                           2      100         100           100                                           4      100         100           100                                           8      100         90            100                                           16     90          90            100                                           24     90          70            100                                           ______________________________________                                    

The paints tested are sufficiently representative that there is a reasonable basis to believe that the primer of the present invention may be used to provide a primer for painting elastomeric silicones using flat latex paint, semi-gloss latex paint, gloss latex paint, flat oil paint, semi-gloss oil paint, or gloss oil paint. The primer of the present invention is particularly susceptible to moisture and hydrolyzes or cures almost immediately upon exposure to trace quantities of water. 

Having described the invention that which is claimed is:
 1. A primer composition for room temperature vulcanizable silicone elastomers consisting essentially of a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula:

    (NH.sub.2 (CH.sub.2).sub.2 NH(CH.sub.2).sub.3)Si(OCH.sub.3).sub.2 O--).sub.2 --(Si(CH.sub.3).sub.2 O--).sub.x

where x varies from about 6 to about 12 whereby cured room temperature vulcanizable silicone elastomers may be painted after said primer has been applied to said cured silicone elastomer.
 2. A paintable article comprising a cured elastomeric silicone and a primer consisting essentially of a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula (NH₂ (CH₂)₂ NH(CH₂)₃)Si(OCH₃)₂ O--)₂ --(Si(CH₃)₂ O--)_(x) where x varies from about 6 to about 12 wherein said primer has been applied to said elastomeric silicone.
 3. The article of claim 2 further comprising a paint selected from the group of paints consisting of flat latex paint, semi-gloss latex paint, gloss latex paint, flat oil paint, semi-gloss oil paint, and gloss oil paint wherein said paint has been applied to said article.
 4. A method of painting an elastomeric silicone substrate comprising:(a) applying to the elastomeric silicone substrate a primer consisting essentially of a low viscosity dimethylsiloxane oligomer end capped with amino-ethyl-amino-propyl-dimethoxysiloxyl having the formula (NH₂ (CH₂)₂ NH(CH₂)₃)Si(OCH₃)₂ O--)₂ --(Si(CH₃)₂ O--)_(x) where x varies from about 6 to about 12; (b) curing the primer to produce a primed substrate wherein said primer coats said elastomeric substrate, and (c) applying a paint selected from the group consisting of flat latex paint, semi-gloss latex paint, gloss latex paint, flat oil paint, semi-gloss oil paint, and gloss oil paint to the primed substrate. 